I have only once dealt with serious battery damage. And it was all my own fault. I bought this game and was so excited about finally owning one that I only played and played without ever checking the batteries.
Then the machine started acting strange. Strange sounds and dot matrix messed up. Game resetting all that stuff. That's when I discovered the acid all over the cpu (isn't it great how the designers at Williams managed to get the batteries right above the cpu?)
Anyway, it took me some desoldering and cleaning with toothbrush and vinegar but the game booted up again and played for years afterwards.
Well, I can tell you that only happened to me once. Which game this was? A near mint Medieval Madness. Please don't laugh.